The initial discovery of x-rays by Röntgen in 1895 [W. C. Röntgen, “Eine Neue Art von Strahlen (Würzburg: Verlag und Druck der Stahel'schen K. Hof-und Universitäts-Buch-und Kunsthandlung, Würzburg, Germany, 1896); “On a New Kind of Rays,” Nature, Vol. 53, pp. 274-276 (Jan. 23 1896)] was in the form of shadowgraphs, in which the contrast of x-ray transmission for biological samples (e.g. bones vs. tissue) allowed internal structures to be revealed without damaging the samples themselves. However, because of their short wavelength (10 to 0.01 nm, corresponding to energies in the range of 100-100,000 eV), and the absence of materials for which the refractive index for x-rays differs significantly from 1, there are no easy equivalents to refractive or reflective optical elements so commonly used in optical system design. So, even now, the most common use of x-rays is still as a simple shadowgraph, observing the structure of bones and teeth in the offices of doctors and dentists.
Early x-ray “microscopy,” developed more than 50 years after the initial discovery of x-rays, simply consisted of elaborate shadowgraph apparatus, in which the diverging x-rays cast a shadow larger than the object [S. P. Newberry and S. E. Summers, U.S. Pat. No. 2,814,729]. With the advent of computer data collection, it became possible to gather more information from the specimen, changing the relative positions and illumination angles of the x-ray source and specimen in a systematic way. Using multiple transmission measurements taken at multiple angles around the specimen, images can be synthesized by computer that represent a 2-dimensional or 3-dimensional model of the specimen [G. N. Hounsfield, U.S. Pat. No. 3,778,614]. The “slices” of interior bodies so revealed are amazing to look at, revealing a great deal about the internal structures without invasive surgery. However, as far as the physics of the x-ray interaction with the specimen, these tomographic reconstructions represent nothing more than an elaborate map of x-ray absorption—a sophisticated shadowgraph.
Over time, other imaging tools for x-ray optical systems were invented. Apparatus using grazing incidence reflection off of surfaces provided cone reflectors [C. G. Wang, U.S. Pat. No. 4,317,036] and capillary collimators [F. Kumasaka et al., U.S. Pat. No. 5,276,724] to allow a diverging x-ray beam to be manipulated into a collimated beam or to concentrate x-rays onto a specimen.
With the development of high-resolution patterning with electron-beam lithography in the 1970's, Fresnel zone plates, which use diffractive properties to effectively focus an electromagnetic wave, could now be manufactured at the small dimensions suitable for use with short x-ray wavelengths. [J. Kirz, “Phase zone plates for x rays and the extreme uv”, Journal of the Optical Society of America, Vol. 64(3), pp. 301-309 (March 1974)]. Zone plates can be used both to shape and focus the illuminating optics and also to collect and focus the transmitted x-rays onto a detector [G. Schmahl and D. Rudolph, “X-Ray Microscopy” pp. 192-202, (Springer Verlag, Berlin, 1984); and U.S. Pat. No. 4,870,674]. Variations using phase-contrast rings [G. Schmal [sic] and D. Rudolph, U.S. Pat. No. 5,550,887] have been developed, and are now commonly used in contemporary x-ray microscopes.
Unfortunately, what a zone plate microscope design may have in resolution may not be matched in imaging speed. The diffractive properties of the zone plate are tuned to a specific wavelength, meaning that most of the energy in a broad-band x-ray source is discarded. Synchrotron sources may increase brightness for a particular wavelength, but are not suitable for portable systems and, at the selected wavelength, the best diffraction efficiency that can be achieved is still under 35%.
Because of this, the microscopy of specimens requiring high speed and high resolution use electron microscopy instead, either as scanning electron microscopes (SEMs) or transmission electron microscopes (TEMs). Being charged particles, electrons can be easily controlled and focused using electric and magnetic fields, and the science and technology of electron optics is a well-developed and established field. [L. Reimer, “Electron Optics”, Section 2 of Ch. 2 of “Scanning Electron Microscopy: Physics of Image Formation and Microanalysis, 2nd Edition”, (Springer Verlag, Heidelberg, 1998)].
Electron beams require that the sample and the beam path must all be in a vacuum. Since any sample would lose all its water in the desiccating environment of a vacuum chamber, this does not represent a way of observing most biological samples in their “natural” condition. Also, depending on their energy, electrons tend to be absorbed with the first few nanometers of a sample, making them extremely useful for the observation of surfaces, but not so useful for the observation of internal structures. Samples must be thinned to be less than 100 nm thick, and often only a few tens of nm thick, before they can be used in a TEM.
In an attempt to combine the penetrating power of x-rays with the control and resolution possible with electron-beams, a hybrid of x-ray microscopy and photoemissive electron microscopy, or PEEM, has been developed [O. H. Griffith and W. Engel, “Historical perspective and current trends in emission microscopy, mirror electron microscopy and low-energy electron microscopy,” Ultramicroscopy, Vol. 36, p. 1 (1991)]. Although PEEM is usually a technique in which a surface is excited from the front and photoelectrons also emitted from the same front surface, a photocathode mounted on a sufficiently thin membrane can allow excitation from the back side through a membrane [H. Hirose, U.S. Pat. No. 5,045,696].
FIG. 1 illustrates a prior art hybrid x-ray/PEEM system as disclosed by F. Cerrina and T. B. Lucatorto on Drawing Sheet 2 of U.S. Pat. No. 6,002,740. In this system, described as being a system to inspect masks for x-ray lithography, the mask 22 is placed between a source of x-rays 30 and converter 18 comprising a photo-emitting cathode 16 mounted on a membrane 19. When the converter 18 is illuminated through the membrane 19 by x-rays, it emits electrons 32 whose intensity is “directly proportional to the local intensity of the x-rays impinging thereon.”
The electrons 32 emitted from the converter 18 are then highly magnified by a set of electron optics in the electron microscope 17. The electron microscope 17 forms an image of the mask pattern that may be fed to the computer system 20 for analysis and display.
The Cerrina disclosure describes a hybrid x-ray/PEEM inspection system for x-ray lithography masks, in which the system emulates an x-ray lithography system. [H. Smith and M. Schattenberg, “X-ray lithography from 500 to 30 nm,” IBM Journal of Research and Development, Vol. 37(3), p. 319 (1993)]. The configuration described requires placing the photoemitting cathode relative to the mask in the same position that a photoresist-coated wafer would be placed in an x-ray lithography system, allowing the image to mimic what the mask would print. In such a lithography system, both the mask and the wafer are placed in a vacuum in close proximity for proximity printing, with a distance of less than 25 microns separating them to minimize distortions, [A. D. Dubner et al., “Diffraction effects in x-ray proximity printing,” Journal of Vacuum Science and Technology B, Vol. 10(5), pp. 2234-2242 (1992)] but not in direct contact to avoid damaging the mask or wafer.
Such hybrid systems were proposed but never applied to x-ray lithographic mask inspection because x-ray lithography did not achieve any widespread commercial adoption. Such systems have been built and demonstrated for various biological and mineral samples. [R. N. Watts et al., “A transmission x-ray microscope based on secondary-electron imaging,” Review of Scientific Instruments, Vol. 68, p 3464 (1997); G. De Stasio et al., “Soft-x-ray transmission photoelectron spectromicroscopy with the MEPHISTO system,” Review of Scientific Instruments, Vol. 69, p. 3106 (1998), and “MEPHISTO spectromicroscope reaches 20 nm lateral resolution,” Review of Scientific Instruments, Vol. 70, p. 1740 (1999); Y. Hwu et al., “Using photoelectron microscopy with hard x-rays,” Surface Science, Vol. 480, pp. 188-195 (2001)]. However, many biological structures are well observed by variations of conventional optical and x-ray tomographic tools, making the complexity of these hybrid systems unnecessary for many biological applications.
But, for one particular class of specimens, variations on this hybrid technique may be perfectly suited, and are the subject of the invention disclosed here.
One problem that has recently emerged is the need to examine products containing integrated devices, such as integrated circuits (ICs), to verify that the devices have been manufactured as specified. This is especially important when the security and integrity of the devices may be an issue, in which is it necessary to insure that additional circuitry (e.g. RF antennas to relay signals from unauthorized sources) have not been inserted during the manufacturing process. When all circuit structures are encased within a single package, verification of the actual contents of the circuit is difficult.
Current examination techniques for these circuit packages require destructive testing, taking the circuit package and removing material layer by layer, photographing and analyzing the circuit patterns of each layer as they are exposed with either an optical microscope, or with an electron microscope for smaller structures. This can be very tedious and time consuming. With the components of the most modern ICs quickly approaching 20 nm in size, and potentially becoming as small as 5 nm in future generations, there is a real need for an imaging technique which has the resolution to identify these small features and also the speed to observe multiple layers of devices and interconnects over a 1 cm by 1 cm area in a manageable amount of time.
An approach using the transmissive power of x-rays to examine the internal contents of a circuit will not require the destruction of the circuit itself, and has the potential to provide both the resolution needed and the speed required.
Systems using an x-ray microscope for the inspection of integrated circuits have been disclosed by the Xradia Corporation [W. Yun and Y. Wang, U.S. Pat. No. 7,119,953; Y. Wang et al., U.S. Pat. No. 7,394,890; M. Bajura et al., U.S. Pat. No. 8,139,846; <http://www.xradia.com/>]. FIG. 2 illustrates a prior art x-ray microscope system as disclosed on Drawing Sheet 2 of U.S. Pat. No. 7,119,953. In such a system, x-rays from a source 1110 are collected by a condenser 1120, which relays x-rays from the source 1110 to the test object 1010. This condenser 1120 is described in some embodiments as a capillary condenser with a suitably configured reflecting surface, while in others as a zone plate. The converging beam from the condenser 1120 irradiates the test object 1010, and the radiation emerging from the test object 1010 is scattered and diffracted out of the path of the direct radiation beam. An objective 1118 is therefore used to form an image of the object, collecting the scattered x-rays. This objective 1118 is described as being possibly a zone plate lens, a Wolter optic, or a Fresnel optic. In some embodiments, an additional phase plate 1116, often in the form of a ring around the center axis of the system, is included to enhance contrast. Both the phase plate 1116 and the objective 1118 are described as being attached to a “high-transmissive substrate” 1140 to form a composite optic 1138. The image of the test object 1010 is formed on a detector 1125, which is described as possibly comprising in some embodiments a charged coupled device (CCD), and in some embodiments comprising a scintillator, and in others being a film-based detector.
X-ray systems with Fresnel zone plate (FZP) optics such as this prior art Xradia system can be effective for the non-destructive examination of integrated circuits, but the limitations of the zone plate optics [J. Kirz and D. Attwood, “Zone Plates”, Sec. 4.4 of the “X-ray Data Booklet” <http://xdb.lbl.gov/Section4/Sec—4-4.html>] reduce the wavelength range over which x-rays can be effectively collected, and increase the time to collect data for a complete IC. The system is therefore very slow and inefficient for collecting large volumes of data on multiple layers of an IC.
There is therefore a need for a system that can combine the penetrating power of x-rays with the easy control possible in electron imaging, and in particular for the application to the microscopy of sub-100 nm structures in integrated circuits to allow rapid, non-destructive testing and inspection of those integrated circuits.